Method and device for performing random access process in unlicensed band

ABSTRACT

Provided are a method for performing a random access process in an unlicensed band, and a device using the same. The device receives a random access preamble (RAP) order instructing the transmission of a RAP. The device performs a clear channel assessment (CCA) in an unlicensed cell during an RAP window so as to transmit the RAP when the device succeeds in CCA.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to wireless communication, and moreparticularly, to a method of performing a random access process in anunlicensed band, and a device using the method.

Related Art

With the explosive increase in mobile data traffic in recent years, aservice provider has utilized a wireless local area network (WLAN) todistribute the data traffic. Since the WLAN uses an unlicensed band, theservice provider can address a demand for a significant amount of datawithout the cost of an additional frequency. However, there is a problemin that an interference phenomenon becomes serious due to a competitiveWLAN installation between the providers, quality of service (QoS) cannotbe guaranteed when there are many users, and mobility cannot besupported. As one of methods for compensating this, a long termevolution (LTE) service in the unlicensed band is emerged.

LTE in unlicensed spectrum (LTE-U) or licensed-assisted access using LTE(LAA) is a technique in which an LTE licensed band is used as an anchorto combine a licensed band and an unlicensed band by the use of carrieraggregation (CA). A user equipment (UE) first accesses a network in thelicensed band. A base station (BS) may offload traffic of the licensedband to the unlicensed band by combining the licensed band and theunlicensed band according to a situation.

The LTE-U may extend an advantage of LTE to the unlicensed band toprovide improved mobility, security, and communication quality, and mayincrease a throughput since the LTE has higher frequency efficiency thanthe legacy radio access technique.

Unlike the licensed band in which exclusive utilization is guaranteed,the unlicensed band is shared with various radio access techniques suchas the WLAN. Therefore, each communication node acquires a channel to beused in the unlicensed band in a contention-based manner, and this iscalled a carrier sense multiple access with collision avoidance(CSMA/CA). Each communication node must perform channel sensing beforetransmitting a signal to confirm whether a channel is idle, and this iscalled clear channel assessment (CCA).

At present, an LTE-UE supports only downlink transmission in theunlicensed band. However, in order to provide more various services,there is a need to consider uplink transmission.

SUMMARY OF THE INVENTION

The present invention provides a method of performing a random accessprocess in an unlicensed band, and a device using the method.

In an aspect, a method for performing a random access process in anunlicensed band is provided. The method includes receiving, by awireless device, a random access preamble (RAP) order instructing atransmission of an RAP, performing, by the wireless device, clearchannel assessment (CCA) during an RAP window in an unlicensed cell, andif the CCA is successful, transmitting the RAP.

The method may further includes receiving, by the wireless device, arandom access response (RAR) comprising a timing advance command (TAC)in response to the RAP, and adjusting, by the wireless device, uplink(UL) time alignment in the unlicensed cell based on the TAC and a timepoint at which the RAP is transmitted.

In another aspect, a device for performing a random access process in anunlicensed band includes a transceiver configured to transmit andreceive a radio signal, and a processor operatively coupled to thetransceiver. The processor is configured to instruct the transceiver toreceive a random access preamble (RAP) order instructing a transmissionof an RAP, instruct the transceiver to perform clear channel assessment(CCA) during an RAP window in an unlicensed cell, and instruct thetransceiver to transmit the RAP if the CCA is successful.

A random access process for uplink transmission can be performed in anunlicensed band.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a long term evolution (LTE) service using anunlicensed band.

FIG. 2 shows an example of a frame based equipment (FBE)-based listenbefore talk (LBT) operation.

FIG. 3 shows an example of a load based equipment (LBE)-based LBToperation.

FIG. 4 is a flowchart showing a random access process according to theconventional technique.

FIG. 5 shows a random access method according to an embodiment of thepresent invention.

FIG. 6 shows an example of a random RAP transmission scheme.

FIG. 7 shows another example of a random RAP transmission scheme.

FIG. 8 shows one example of an RAP transmission re-attempt.

FIG. 9 is a block diagram showing a wireless communication systemaccording to an embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

A wireless device may be fixed or mobile, and may be referred to asanother terminology, such as a user equipment (UE), a mobile station(MS), a mobile terminal (MT), a user terminal (UT), a subscriber station(SS), a personal digital assistant (PDA), a wireless modem, a handhelddevice, etc. The wireless device may also be a device supporting onlydata communication such as a machine-type communication (MTC) device.

A base station (BS) is generally a fixed station that communicates withthe wireless device, and may be referred to as another terminology, suchas an evolved-NodeB (eNB), a base transceiver system (BTS), an accesspoint, etc.

Hereinafter, it is described that the present invention is appliedaccording to a 3rd generation partnership project (3GPP) long termevolution (LTE) based on 3GPP technical specification (TS). However,this is for exemplary purposes only, and thus the present invention isalso applicable to various wireless communication networks.

In a carrier aggregation (CA) environment or a dual connectivityenvironment, the wireless device may be served by a plurality of servingcells. Each serving cell may be defined with a downlink (DL) componentcarrier (CC) or a pair of a DL CC and an uplink (UL) CC.

The serving cell may be classified into a primary cell and a secondarycell. The primary cell operates at a primary frequency, and is a celldesignated as the primary cell when an initial network entry process isperformed or when a network re-entry process starts or in a handoverprocess. The primary cell is also called a reference cell. The secondarycell operates at a secondary frequency. The secondary cell may beconfigured after an RRC connection is established, and may be used toprovide an additional radio resource. At least one primary cell isconfigured always. The secondary cell may be added/modified/released byusing higher-layer signaling (e.g., a radio resource control (RRC)message).

A cell index (CI) of the primary cell may be fixed. For example, alowest CI may be designated as a CI of the primary cell. It is assumedhereinafter that the CI of the primary cell is 0 and a CI of thesecondary cell is allocated sequentially starting from 1.

FIG. 1 shows an example of an LTE service using an unlicensed band.

A wireless device 130 establishes a connection with a 1st BS 110, andreceives a service through a licensed band. For traffic offloading, thewireless device 130 may receive a service through an unlicensed bandwith respect to a 2nd BS 120.

The 1st BS 110 is a BS supporting an LTE system, whereas the 2nd BS 120may also support other communication protocols such as a wireless localarea network (WLAN) in addition to LTE. The 1st BS 110 and the 2nd BS120 may be associated with a carrier aggregation (CA) environment, and aspecific cell of the 1st BS 110 may be a primary cell. Alternatively,the 1st BS 110 and the 2nd BS 120 may be associated with a dualconnectivity environment, and a specific cell of the 1st BS 110 may be aprimary cell. In general, the 1st BS 110 having the primary cell haswider coverage than the 2nd BS 120. The 1st BS 110 may be called a macrocell. The 2nd BS 120 may be called a small cell, a femto cell, or amicro cell. The 1st BS 110 may operate the primary cell and zero or moresecondary cells. The 2nd BS 120 may operate one or more secondary cells.The secondary cell may be activated/deactivated by an indication of theprimary cell.

The above description is for exemplary purposes only. The 1st BS 110 maycorrespond to the primary cell, and the 2nd BS 120 may correspond to thesecondary cell, so that the cell can be managed by one BS.

The licensed band is a band in which an exclusive use is guaranteed to aspecific communication protocol or a specific provider.

The unlicensed band is a band in which various communication protocolscoexist and a shared use is guaranteed. The unlicensed band may include2.5 GHz and/or 5 GHz band used in a WLAN.

It is assumed in the unlicensed band that a channel is occupiedbasically through contention between respective communication nodes.Therefore, in communication in the unlicensed band, it is required toconfirm that signal transmission is not achieved by other communicationnodes by performing channel sensing. For convenience, this is called alisten before talk (LBT), and if it is determined that signaltransmission is not achieved by other communication nodes, this case isdefined as confirmation of clear channel assessment (CCA).

The LBT must be performed preferentially in order for a BS or wirelessdevice of an LTE system to have access to a channel in the unlicensedband. Further, when the BS or wireless device of the LTE systemtransmits a signal, an interference problem may occur since othercommunication nodes such as the WLAN or the like also perform the LBT.For example, in the WLAN, a CCA threshold is defined as −62 dBm as to anon-WLAN signal and is defined as −82 dBm as to a WLAN signal. Thismeans that interference may occur in an LTE signal due to other WLANdevices when the LTE signal is received with power less than or equal to−62 dBm.

Hereinafter, when it is said that ‘LBT is performed’ or ‘CCA isperformed’, it implies that whether a channel is idle or is used byanother node is confirmed first and thereafter the channel is accessed.

Hereinafter, the LTE and the WLAN are described for example as acommunication protocol used in the unlicensed band. This is forexemplary purposes only, and thus it may also be said that a 1stcommunication protocol and a 2nd communication protocol are used in theunlicensed band. A BS supports the LTE. A UE is a device supporting theLTE.

Hereinafter, although it is described that downlink (DL) transmission isbased on transmission performed by a BS and uplink (UL) transmission isbased on transmission performed by a UE, the DL transmission and the ULtransmission may also be performed by a transmission node or node groupin a wireless network. The UE may imply an individual node which existsfor each user, and the BS may imply a central node fortransmitting/receiving and controlling data for a plurality ofindividual nodes. From this perspective, the term ‘BS’ may be replacedwith a DL node, and the term ‘UE’ may be replaced with a UL node.

Hereinafter, a cell (or a carrier) operating in an unlicensed band iscalled an unlicensed cell or an unlicensed carrier. A cell operating ina licensed band is called a licensed cell or a licensed carrier.

An LBT operation in an unlicensed band is regulated in some countries.For example, in Europe, there are two types of LBT operations which arenamed as frame based equipment (FBE) and load based equipment (LBE).

FIG. 2 shows an example of an FBE-based LBT operation.

A channel occupancy time implies a time during which transmission can becontinued when a communication node successfully accesses a channel, andhas a value of about 1 ms to 10 ms. A frame is an idle timecorresponding to at least 5% of the channel occupancy time. CCA isdefined as an operation of observing the channel during at least 20 μsbefore an end portion within the idle time. The communication nodeperiodically performs the CCA in unit of the frame, and transmits dataduring a next channel occupancy time in a state where the channel is notoccupied. The communication node defers transmission in a state wherethe channel is occupied, and waits until a next frame.

The FBE-based LBT operation implies that a channel occupancy time and achannel detection time are predefined, and CCA is performed only at adetermined channel detection time, and may be called a fixed CCAexecution method.

FIG. 3 shows an example of an LBE-based LBT operation.

An idle time is defined between channel occupancy times. The idle timeis divided into a plurality of slots. A communication node may first seta value of qϵ{4, 5, . . . , 32} and thereafter perform CCA for one slot,and if a channel is in an unoccupied state in the CCA slot, may transmitdata by ensuring a channel occupancy time having a length of (13/32)qms. If the channel is in an occupied state in the first CCA slot, thecommunication node randomly chooses a value of Nϵ{1, 2, . . . , q} andstores it as an initial value of a backoff counter. Thereafter, if thechannel is in the unoccupied state in one CA slot while sensing achannel state in unit of the CCA slot, a value of the backoff counter isdecreased by 1. If the value of the backoff counter becomes 0, thecommunication node may transmit data during the channel occupancy timehaving the length of (13/32)q ms.

The LBE-based LBT operation implies that the communication node performsthe CCA by randomly determining a channel detection time (e.g., the CCAslot) according to whether the channel is occupied, and may be called arandom CCA execution method.

Hereinafter, a parameter for selecting an LBT method and determining abackoff length, a parameter for determining a size of a CCA slot, and aparameter used for an LBT operation such as a period and time offset fordetermining CCA timing are called an LBT parameter. A unit of signalstransmitted by a radio node through an LBT operation performed one timeis called a transmission burst.

FIG. 4 is a flowchart showing a random access process according to theconventional technique. The random access process is used by a UE for ULsynchronization acquisition or UL radio resource allocation.

A UE receives a root index and a physical random access channel (PRACH)configuration index from a BS. Each cell has 64 candidate random accesspreambles defined by a Zadoff-Chu (ZC) sequence. The root index is alogical index for generating the 64 candidate random access preambles bythe UE.

The random access preamble is limited to a specific time and frequencyresource for each cell. The PRACH configuration index indicates aspecific subframe and preamble format capable of transmitting the randomaccess preamble.

The UE transmits a randomly selected random access preamble to the BS(S110). The UE selects one of the 64 candidate random access preambles.In addition, the UE selects a corresponding subframe by using the PRACHconfiguration index. The UE transmits the selected random accesspreamble in the selected subframe.

Upon receiving the random access preamble, the BS transmits a randomaccess response (RAR) to the UE (S120). The RAR is detected in twosteps. First, the UE detects a PDCCH masked with a random access-RNTI(RA-RNTI). The UE receives the RAR included in a medium access control(MAC) protocol data unit (PDU) through a PDSCH indicated by the detectedPDCCH.

The RAR may include a timing advance command (TAC), a UL grant, and atemporary C-RNTI. The TAC is information indicating a time alignmentvalue sent by a BS to a UE to maintain a UL time alignment. The UEupdates UL transmission timing by using the time alignment value. Whenthe UE updates the time alignment, a time alignment timer starts orrestarts. The UE can perform a UL transmission only when the timealignment timer is running.

Upon receiving the random access response, the UE transmits a scheduledmessage to the BS according to a UL grant included in the RAR (S130).

Meanwhile, a random access preamble (RAP) may be transmitted to confirma UL synchronization and UL channel state of each UE also in anunlicensed band. A method of transmitting the RAP in the unlicensed bandis proposed.

Hereinafter, a radio frame includes 10 subframes. One subframe mayinclude a plurality of orthogonal frequency division multiplexing (OFDM)symbols in a time region. A time required to transmit one subframe iscalled a transmission time interval (TTI). For example, 1 TTI may be 1ms. The OFDM symbol is only for expressing one symbol period in the timeregion, and there is no limitation in a multiple access scheme orterminologies. For example, the OFDM symbol may also be referred to asanother terminology such as a single carrier-frequency division multipleaccess (SC-FDMA) symbol, a symbol period, etc.

FIG. 5 shows a random access method according to an embodiment of thepresent invention.

A BS transmits to a UE an RAP order for instructing RAP transmission inan unlicensed cell. The RAP order may be transmitted in a licensed cell(e.g., a primary cell), or may be transmitted in an unlicensed cell(e.g., a secondary cell).

The RAP order may be instructed by downlink control information (DCI)transmitted through a DL control channel (e.g., a PDCCH or an EPDCCH).In the unlicensed cell, whether the UE will transmit an RAP at any timepoint depends on a CCA result. In order for the UE to increase anopportunity of transmitting the RAP, the RAP order may includeinformation regarding a time/frequency region in which the UE cantransmit the RAP.

The following table exemplifies information included in the RAP order.Not all information is essential, and thus another information may beadded and some information may be omitted.

TABLE 1 Field name Description Cell index This indicates an unlicensedcell in which an RAP will be transmitted, and may be omitted when an RAPorder is transmitted in the unlicensed cell. RAP resource This indicatesa preamble index and/or frequency resource for an RAP. RAP window Thisindicates a start point and/or size of an RAP window in whichtransmission of an RAP will be attempted.

The RAP window may include successive subframes in which the RAP will betransmitted. For example, if the RAP order is received in a subframe n,a subframe n+k1 to a subframe n+k1+k2 may correspond to the RAP window.‘k1’ may be a start time of the RAP window, and ‘k2’ may be a length ofthe RAP window. The RAP order may include information regarding k1 andk2. The UE may perform CCA from the subframe n+k1, and may transmit theRAP when an RAP resource is idle.

Information regarding the RAP resource and the RAP window may bepredetermined or may be configured through an additional message (e.g.,a radio resource control (RRC) message), instead of the RAP order.

A plurality of RAP resources may be configured within one RAP window.When the subframe n+k1 to the subframe n+k1+k2 correspond to the RAPwindow, the UE may use RAP resources starting from an RAP resourceclosest to the subframe n+k1 in RAP transmission. Alternatively, it isassumed that the number of RAP resources is ‘r1’. The UE may use r1 RAPresources starting from the RAP resource closest to the subframe n+k1.It is assumed that the number of subframes in which the RAP resource isconfigured is ‘r2’. The UE may use r2 subframes starting from thesubframe n+k1 in RAP transmission. Alternatively, the RAP order maydesignate one of the plurality of RAP windows and/or one of theplurality of RAP resources.

The UE may transmit the RAP only one time in the RAP window, or even ifRAP transmission is successful, may re-attempt RAP transmission byrepeating the CCA. The UE may transmit the RAP multiple times during theRAP window.

A DL control channel for carrying the RAP order may be transmitted inthe unlicensed cell. In order for this DL control channel to have ahigher transmission priority, a shorter backoff counter or a smaller CCAslot may be applied in comparison with other channels. Alternatively, aCCA threshold to be applied to CCA of the RAP order may use a highervalue in comparison with other channels.

The UE which has received the RAP order performs CCA during the RAPwindow, and if a channel is idle, transmits the RAP.

When performing the CCA for the RAP, it may be determined that the CCAis successful if a shorter maximum backoff counter is configured incomparison with other UL channels, or if the CCA is successful even onlyone time during a short CCA slot. Alternatively, the CCA threshold to beapplied to RAP transmission may use a higher value in comparison withother UL channels.

The UE may perform the CCA only for a frequency region assigned to RAPtransmission during the RAP window.

A subframe designated for a specific usage may be excluded from the RAPwindow. The RAP window may include a subframe excluding a subframe whichcannot be used in UL transmission or RAP transmission (e.g., a durationdesignated for transmission of a DL discovery reference signal (DRS)).

If an RAP transmission duration corresponds to n OFDM symbols in asubframe, RAP transmission may be limited to last n OFDM symbols in thesubframe for the CCA operation.

The following operation is possible for an exact time point at which theUE can transmit the RAP.

In one exemplary embodiment, a time point at which the UE can start RAPtransmission may be limited only to a boundary of a subframe or aboundary of an OFDM symbol. This is called a limited RAP transmissionscheme. The UE may transmit a reservation signal defined separately fromthe RAP in order to occupy a channel between time points at which RAPtransmission is allowed after CCA is successful.

In another embodiment, the UE may start RAP transmission at any timepoint at which the CCA operation is complete within a given timeduration. This is called a random RAP transmission method. Since the BScannot know an RAP transmission time point of the UE, the UE and the BSneed to share a predefined reference time point.

FIG. 6 shows an example of a random RAP transmission scheme.

It is assumed that a subframe has a length of 1 ms, and a reference timepoint is given in unit of subframes. It is assumed that two referencetime points t_ref1 and t_ref2 are defined in one subframe. The number ofreference time points and positions thereof are for exemplary purposesonly.

CCA is successful after t_ref1, and thus a UE transmits an RAP. Anoffset t_tx is defined between a reference time point and a time atwhich the RAP is actually transmitted. It is assumed that t_tx=0.1 ms.Thereafter, the UE receives TAC for correcting a UL transmission timefrom a BS. The BS calculates the TAC by regarding that the UE transmitsthe RAP at the reference time point, and thus the UE can correct the TACby using the offset. If the TAC is 0.15 ms, a time alignment value usedactually by the UE in UL transmission is TAC-t_tx=0.05 ms. That is, evenif the BS instructs to advance UL transmission timing by the TAC, the UEadvances the UL transmission timing by TAC-t_tx.

FIG. 7 shows another example of a random RAP transmission scheme.

CCA is successful after t_ref2, and thus a UE transmits an RAP. It isassumed that an offset between a reference time point and a time atwhich the RAP is actually transmitted is t_tx=0.05 ms. Thereafter, theUE receives TAC for correcting a UL transmission time from a BS. If theTAC is 0.15 ms, a time alignment value used actually by the UE in ULtransmission is TAC-t_tx=0.1 ms.

A plurality of reference time points may be defined within an RAPwindow. In order for the BS to be able to identify specific referencetime points when a time point at which the UE transmits the RAP existsbetween the specific reference time points, a time point at which the UEcan transmit the RAP may be limited to a certain duration between tworeference time points.

A limited RAP transmission scheme and a random RAP transmission schememay be combined. One or more time durations in which the UE can startRAP transmission in the RAP window is determined, and the random RAPtransmission scheme is applied in a corresponding time duration.

Which one will be applied between the limited RAP transmission schemeand the random RAP transmission scheme may be informed to the UE by theBS through an RAP order or an RRC message.

The UE may fail to transmit the RAP due to a failure in CCA during theRAP window. In this case, the UE may re-attempt RAP transmission or maydeclare a failure in the RAP transmission.

FIG. 8 shows one example of an RAP transmission re-attempt.

If a channel is busy during a first RAP window and thus RAP transmissionfails, a UE attempts RAP transmission during a second RAP window. Thesecond RAP window may be consecutive with the first RAP window or mayappear after a time offset t_wait. The time offset t_wait may bepredetermined or randomly acquired value. A size of an i-th RAP windowmay be predetermined or may be increased or decreased according to aspecific rule.

If the RAP is not transmitted due to a failure in CCA, transmit power ofthe RAP to be retransmitted is not increased. If the RAP window isapplied, RAP transmit power may be set identically in all RAP windows.That is, even if the RAP is not transmitted due to a failure in the CCAin the first RAP window and thus RAP transmission is performed in thesecond RAP window, the RAP transmit power is not increased. This is tomaintain constant transmit power so that other nodes performing the CCAcan detect the RAP within certain coverage. However, even if the RAP istransmitted in a previous RAP window, the RAP may not be received, andthus RAP transmit power may be increased when retransmission of the RAPis attempted in a next RAP window.

If the CCA threshold is defined to be inverse proportional to the RAPtransmit power, transmit power of the next RAP window may be decreasedto be lower than transmit power of the previous RAP window in order toavoid a continuous RAP transmission failure caused by a CCA failure.

If the CCA is not successful during one or more RAP windows, the UE maydiscard RAP transmission. Alternatively, the UE may discard RAPtransmission in case of a failure in reception of an RAR to be describedbelow. If the RAP transmission is discarded, the UE may transmitinformation regarding a reason of discarding to the BS. This informationmay be transmitted from a licensed cell or an unlicensed cell through anRRC message or the like. If it is possible to distinguish whether theRAP transmission is discarded due to a failure in CCA or whether the RAPis transmitted but RAR reception fails, it may be useful to determinewhether the BS will instruct again an RAP order in a correspondingunlicensed cell. In addition, if the UE fails in RAP transmission due toincompletion of the CCA operation during a given time duration,measurement information (e.g., reference signal received power(RSRP)/received signal strength indicator (RSSI)/interferenceinformation, etc.) regarding a corresponding unlicensed cell may beprovided to the BS.

The UE may provide the BS with the number of CCA attempts for RAPtransmission, the number of times of transmitting the RAP due tosuccessful CCA transmission, or statistics on a ratio thereof. Thestatistics may be transmitted in the license cell.

Now, RAR reception will be described.

After transmitting an RAP in an unlicensed cell, a UE attempts RARreception from a BS during a random access response (RAR) receptionduration. The RAR may be received in the unlicensed cell or may bereceived in a cell in which an RAP order is received.

If the RAR is transmitted in the unlicensed cell, a backoff counter forthe RAR may be shorter in comparison with other cells. In addition, aCCA slot for the RAR may be shorter in comparison with other channels. ACCA threshold to be applied to RAR transmission may use a higher valuein comparison with other channels.

If the RAR is not received during an RAR reception duration, the UE mayattempt RAP retransmission, or may stop the RAP transmission attempt. Ifthe RAR is not received through RRC signaling or the RAP order, the BSmay determine whether to attempt RAP retransmission or stop the RAPtransmission attempt.

If the RAR is transmitted in the unlicensed cell, a longer time may berequired in comparison with RAR transmission without CCA. Therefore, theRAR reception duration in the unlicensed cell may be set to be greaterthan the RAR reception duration in the licensed cell.

The RAR may include at least one of resource allocation for ULtransmission, TAC for correcting time alignment, and transmit powercorrection for adjusting UL transmit power. If a physical uplink sharedchannel (PUSCH) is scheduled by the RAR, the UE may transmit a PUSCH bycorrecting the UL transmit power and/or by applying the TAC. The PUSCHmay be transmitted in the unlicensed cell, which implies that the PUSCHis transmitted after the CCA is complete. Alternatively, the PUSCH maybe transmitted in the licensed cell (e.g., a primary cell) without CCA.This is to prevent the PUSCH from not being able to be transmitted dueto a failure in the CCA. The RAR may include information indicating acell in which the PUSCH will be transmitted. The TAC and/or the ULtransmit power correction within the RAR may be applied only when thePUSCH is transmitted in the unlicensed cell.

A redundancy version (RV) and an HARQ process number for the PUSCHtransmitted in response to the RAR may be designated as a predeterminedvalue. An HARQ process number 0 (i.e., a first HARQ process number) andan RV 0 (i.e., an RV including a coding bit having systematicinformation) may be used under the assumption that the PUSCHtransmission corresponds to initial transmission.

If UL transmission in the unlicensed cell is scheduled, the RAR mayinclude LBT information regarding an LBT parameter (a CCA threshold, abackoff counter, etc.) to be applied to UL transmission. Content of theRAR may differ depending on whether the RAR is a response for RAPtransmission in the licensed cell or a response for RAP transmission inthe unlicensed cell. Alternatively, the response for RAP transmission inthe licensed cell and the response for RAP transmission in theunlicensed cell may be multiplexed in one RAR. The RAR may include anidentifier for indicating whether the RAR is the response for RAPtransmission in the licensed cell or the response for RAP transmissionin the unlicensed cell.

Regarding PUSCH transmission in the unlicensed cell, whethertransmission is possible may be uncertain or may be significantlydelayed depending on a CCA result, and thus the UE may not transmit aPUSCH. The RAR may include information indicating whether to transmitthe PUSCH corresponding to the RAR. If PUSCH transmission is disabled,the RAR may not include UL transmission resource allocation. If thePUSCH transmission is disabled, the UE may transmit a confirmationsignal for informing the BS of RAR reception. This confirmation signalmay be a signal corresponding to HARQ ACK.

FIG. 9 is a block diagram showing a wireless communication systemaccording to an embodiment of the present invention.

A wireless device 50 includes a processor 51, a memory 52, and atransceiver 53. The memory 52 is coupled to the processor 51, and storesvarious instructions executed by the processor 51. The transceiver 53 iscoupled to the processor 51, and transmits and/or receives a radiosignal. The processor 51 implements the proposed functions, procedures,and/or methods. In the aforementioned embodiment, an operation of the UEmay be implemented by the processor 51. When the aforementionedembodiment is implemented with a software instruction, the instructionmay be stored in the memory 52, and may be executed by the processor 51to perform the aforementioned operation.

A BS 60 includes a processor 61, a memory 62, and a transceiver 63. TheBS 60 may operate in an unlicensed band. The memory 62 is coupled to theprocessor 61, and stores various instructions executed by the processor61. The transceiver 63 is coupled to the processor 61, and transmitsand/or receives a radio signal. The processor 61 implements the proposedfunctions, procedures, and/or methods. In the aforementioned embodiment,an operation of the BS may be implemented by the processor 61.

The processor may include Application-Specific Integrated Circuits(ASICs), other chipsets, logic circuits, and/or data processors. Thememory may include Read-Only Memory (ROM), Random Access Memory (RAM),flash memory, memory cards, storage media and/or other storage devices.The RF unit may include a baseband circuit for processing a radiosignal. When the above-described embodiment is implemented in software,the above-described scheme may be implemented using a module (process orfunction) which performs the above function. The module may be stored inthe memory and executed by the processor. The memory may be disposed tothe processor internally or externally and connected to the processorusing a variety of well-known means.

In the above exemplary systems, although the methods have been describedon the basis of the flowcharts using a series of the steps or blocks,the present invention is not limited to the sequence of the steps, andsome of the steps may be performed at different sequences from theremaining steps or may be performed simultaneously with the remainingsteps. Furthermore, those skilled in the art will understand that thesteps shown in the flowcharts are not exclusive and may include othersteps or one or more steps of the flowcharts may be deleted withoutaffecting the scope of the present invention.

What is claimed is:
 1. A method for performing a random access procedurein an unlicensed band, the method performed by a wireless device andcomprising: performing a listen before talk (LBT) in an unlicensed bandto check whether the LBT is failed or not; based on the LBT being notfailed, transmitting a first random access preamble with a firsttransmit power in the unlicensed band; and attempting to transmit asecond random access preamble in the unlicensed band, wherein attemptingto transmit the second random access preamble comprises: based on theLBT being failed, attempting to transmit the second random accesspreamble with a second transmit power in the unlicensed band, the secondtransmit power having a same value with the first transmit power, andbased on the LBT being not failed, attempting to transmit the secondrandom access preamble with a third transmit power in the unlicensedband, the third transmit power having a greater value than the firsttransmit power.
 2. The method of claim 1, wherein performing the LBT inan unlicensed band comprises: sensing a channel during a channeldetection time which is randomly selected by the wireless device;determining that the LBT being not failed based on the channel beingsensed to be idle during the channel detection time; and determiningthat the LBT being failed based on the channel being sensed not to beidle during the channel detection time.
 3. The method of claim 1,further comprising: receiving a random access response as a response tothe first random access preamble or the second random access preamble,the random access response including an uplink resource allocationassigned to a transmission of a scheduled message and a LBT parameter tobe applied to a LBT operation for the transmission of the scheduledmessage.
 4. The method of claim 3, wherein: based on the LBT parameterindicating a first LBT scheme, the scheduled message is transmitted bythe wireless device after a channel is sensed to be idle for apredefined time, and based on the LBT parameter indicating a second LBTscheme, the scheduled message is transmitted by the wireless deviceafter the channel is sensed to be idle for a channel detection timewhich is randomly selected by the wireless device.
 5. A devicecomprising: a processor; and a memory operatively coupled with theprocessor and configured to store instructions that, when executed bythe processor, cause the device to perform functions comprising:performing a listen before talk (LBT) in an unlicensed band to checkwhether the LBT is failed or not; based on the LBT being not failed,transmitting a first random access preamble with a first transmit powerin the unlicensed band; and attempting to transmit a second randomaccess preamble in the unlicensed band, wherein attempting to transmitthe second random access preamble comprises: based on the LBT beingfailed, attempting to transmit the second random access preamble with asecond transmit power in the unlicensed band, the second transmit powerhaving a same value with the first transmit power, and based on the LBTbeing not failed, attempting to transmit the second random accesspreamble with a third transmit power in the unlicensed band, the thirdtransmit power having a greater value than the first transmit power. 6.The device of claim 5, wherein performing the LBT in an unlicensed bandcomprises: sensing a channel during a channel detection time which israndomly selected by the wireless device; determining that the LBT beingnot failed based on the channel being sensed to be idle during thechannel detection time; and determining that the LBT being failed basedon the channel being sensed not to be idle during the channel detectiontime.
 7. The device of claim 5, wherein the functions further comprise:receiving a random access response as a response to the first randomaccess preamble or the second random access preamble, the random accessresponse including an uplink resource allocation assigned to atransmission of a scheduled message and a LBT parameter to be applied toa LBT operation for the transmission of the scheduled message.
 8. Thedevice of claim 7, wherein: based on the LBT parameter indicating afirst LBT scheme, the scheduled message is transmitted by the deviceafter a channel is sensed to be idle for a predefined time, and based onthe LBT parameter indicating a second LBT scheme, the scheduled messageis transmitted by the device after the channel is sensed to be idle fora channel detection time which is randomly selected by the device.